1. Field of the Invention
This invention relates to environmental monitoring and, more particularly, to the monitoring of fine particulate of the PM.sub.2.5 class.
2. Discussion of Related Art
Over the past quarter of a century, government regulations concerning ambient air quality have evolved towards monitoring and regulating ever smaller particles, from the so-called "total suspended particulate" concept, to the PM.sub.10 class, to the "fine particulate" known as PM.sub.2.5.
Among other things, the regulations specify a "reference method" (FRM) to monitor, or measure, the amount of airborne particulate matter in air. The reference method has largely remained unchanged. Typically, the method involves sampling air with particle collection on a filter, combined with gravimetric evaluation of the collected mass. Though this method is responsible for much of the relevant recorded data, it has many drawbacks:
1. It's labor intensive and thus not compatible with long-term continuous unattended operation.
2. It provides time-delayed information and is thus incapable of providing real-time measurements.
3. It's subject to both intrinsic measurement ambiguities as well as operational problems.
Concerning drawback (3), the intrinsic measurement ambiguities result when sampling chemically reactive and/or physically unstable aerosols. Such particles, after their capture on the filter, may evaporate, react, or otherwise be altered with respect to their original airborne condition. These effects are further influenced by the design idiosyncrasies of the FRM sampling device, such as the design's flow velocities, filter structure and composition, internal surface characteristics, wall temperatures and their gradients, etc. Such factors may play a role in influencing the fate of the sampled particulate, even before they are subjected to the required sample conditioning procedures, which may involve exposing the collected sample to a controlled humidity and temperature environment to remove accreted liquid water.
Since the 1970s, two types of quasi-continuous, quasi-real-time ambient particulate monitoring devices have been granted "equivalence status" with respect to the filter/gravimetric reference method: filter tape/beta radiation attenuation mass monitors, and the tapered element oscillating mass monitor (TEOM). (Equivalence status permits these devices to be used as a substitute for a FRM device) Both classes of instruments rely on particle collection by filtration, but each uses a different method of sensing the mass of the collected particulate. The former uses the attenuation of beta rays to quantify that mass, whereas the latter relies on sensing the change in the natural resonant frequency of an oscillating body to which a particle collection filter cartridge is attached. The above-mentioned instruments measure cumulative particulate mass and for typical ambient particulate concentrations require sampling periods of the order of one hour to achieve the necessary measurement sensitivity.
These two classes of quasi-continuous particulate mass monitors suffer from the same intrinsic problems as the reference sampling method. That is, they are based on filtration that may affect the integrity of the particles with respect to their airborne state. Furthermore, to preclude errors resulting from water accretion on particles at high humidity levels, these instruments rely on heating of the sample stream. This method of water removal, however, has been found to affect other volatile particle species of interest. See Obeidi, F. and Eatough, D. J., A Real-Time Monitor for Volatile Fine Particulate Matter, paper presented at 16th Annual Conference of the AAAR, Oct. 13-17, 1997, p. 39. Similar problems are likely to affect water trapping approaches, such as diffusion drying.
All of the above-mentioned mechanisms that affect the validity of ambient particulate monitoring are especially noticeable in the case of fine particles, also called the accumulation mode. This is so, because these particles are predominantly the result of condensation and photochemical processes. These are inherently unstable aerosols, as opposed to the coarse mode which is constituted principally by aerosolized soil grains and other mechanically generated particles. Thus, the validity of any measurements resulting from conventional devices may be considered suspect as the particles may be significantly affected by the measuring devices.
Thus, there is a need for an in situ technique, and corresponding system, to continuously monitor ambient fine particles, ideally in real-time. There is also a need for a system and technique that can monitor particles without affecting any of the particle species. There is yet another need for a system and technique to monitor particles in a manner that is capable of discriminating against liquid water-caused particle growth. All of the above needs are particularly acute for the measurement of fine particulate of the PM.sub.2.5 class.